I was having the "not found" error too after installing Mail through cPanel.

What I think happened was the Mail directory and files were downloaded/installed but the main Mail.php file was not.

So what I did was download the manual installation archive on here. Inside the archive there's a Mail-1.2.0 folder and inside that folder there's Mail.php, a folder named Mail, and a folder named tests.
I went to my server's PHP include directory (/usr/lib/php), and put that main Mail.php file in there.

Please make sure that your php.ini's include_path points to the PEAR directory. If you installed PEAR through the console and had PEAR automatically add its path to php.ini, remember that the added include_path will be available after you have restarted your web server.
It's also important to check the correct case - even on Windows - so it should read require_once("mail.php") without a capital M.